The disposal of streams of waste material is often accomplished in fluidized bed reactors by injecting a waste stream into a fluidized bed of inert particulate material, e.g., beach sand or the like, and burning or incinerating the waste material. Beds made up of such materials provide an enormous surface area for the efficient transfer of heat from the bed particles to the waste material which coats the particles, thereby facilitating the incineration of the waste material. The fluidized bed particles may be kept at the operating temperature for the incineration process by the heat released solely from the combustion reaction between the oxygen in the bed fluidizing gas and the combustible matter in the waste stream. This may be augmented by a gaseous or liquid fuel separately introduced into the bed. The technology for such incineration of waste material is well known and has been practiced in the process industries for many years. For example, it has been utilized for the incineration of waste uranyl nitrate solutions, the disposal of black liquor from paper mills, miscellaneous sludges from petroleum refineries, pharmaceutical plants, and sewage concentrators.
In all of the various forms os such fluidized bed combustors it is imperative that the fluidizing gas be uniformly distributed across the bed area, that the bed material be of such small size that it is fluidized at all times, and that bed particles that become enlarged over time by accretion of non-combustibles in the waste stream be periodically or continuously removed from the bed without requiring that the combustor be shut down.
The precise composition of a waste stream is frequently unknown. This is particularly the case in sewer sludges in which the non-combustible constituents thereof can result in the formation of sticky eutectic mixtures when subjected to the combustion temperatures. Such sticky mixtures can, by adhesion to the bed particles, cause them to grow in size by accumulating layer upon layer of accretions. When such enlarged solids, or agglomerates of the sticky mixtures, reach such a size that they can no longer be sustained in the fluidized state by the fluidizing gas, they sink to the bottom of the bed and lay there as an immobile fixed bed. This then distorts the uniform distribution of the fluidizing gas across the bed area and thereby progressively decreases the efficiency of the combustor. If allowed to progress unchecked, the combustor becomes totally inoperative. Tramp materials in the feed to the bed also result in this undesirable effect. It is therefore of the greatest importance that such oversize components be effectively removed from the bed as expeditiously as possible.
Similar situations arise in fluid beds burning many other materials. For example, many installations burning coal or biomass materials experience operating problems caused by stones in the feed.
In the past, the removal of oversize components from the bottom of a fluidized bed has been accomplished by gently sloping an air distribution grid in a downward direction towards a discharge port in the grid or towards a discharge chute in the wall of the reactor or combustor. With such an arrangement of the grid the mobility of particles within the fluidized bed permits coarser or heavier particles on the grid to migrate towards the discharge port or chute under the grid. Such mobility is primarily dependent on the shape and weight of the solid particles or agglomerates desired to be removed, the superficial velocity of the fluidizing gas, and the slope or grade of the grid in the direction of the discharge port. U.S. Pat. No. 4,253,824 discloses a dual cone distributor grid for removing tramp material together with admixed bed particles from a fluidized bed reactor.
Another means for removing foreign objects, tramp material, oversized inert or otherwise non-combustible material from a fluidized bed is disclosed in U.S. Pat. No. 4,908,124. In this patent, the distributor grid supporting the bed particles is configured to provide a lateral vector of fluidizing gas that urges any oversize objects that have descended to the bottom of the fluidized bed towards a discharge port in the side of the reactor, thereby obviating the need for a sloping distributor grid. U.S. Pat. Nos. 4,421,023 and 4,196,676 also show distributor grids that give the fluidizing gas a lateral vector which tends to move oversize particles at the bottom of the fluidized bed towards a discharge port.
Many of these designs are of a complicated mechanical construction which on heat up or cool down become stressed and eventually fail due to differential thermal expansion of the components.